A CDMA mobile communication system makes it possible to maintain a communication quality at a desired level and to minimize an interference causing to the other users, by performing a transmission power control with a high accuracy. Accordingly, the COMA mobile communication system can increase a radio system capacity. The CDMA mobile communication system includes two steps of a configuration in order to maintain a received SIR (Signal to Interference power Ratio) closer to a target SIR: an inner loop transmission power control and an outer loop transmission power control. In the inner loop transmission power control, a radio base station measures the received SIR and compares the received SIR with the target SIR, and instructs, to a mobile station, to increase or decrease the transmission power by using a transmission power control bit. In the outer loop transmission power control, a radio network controller, which exists as an upper apparatus of the radio base station, measures a reception quality (such as a block error or a block error rate or the like) and instructs, to the radio base station, the target SIR of the radio base station so that the radio base station can satisfy a target quality.
It is preferable that the outer loop transmission power control is performed at every 1[TTI]. For instance, it is ideal that the block error is detected by a CRC and updates the target SIR at every 1[TTI] in the following manner:
IF CRC check OK  Step_down=BLER_target*Step_size  Sir_target(n)=SIR_target(n−1)−Step_downELSE  Step_up=Step_size−BLER_target*Step_size  SIR_target(n)=SIR_target(n−1)+Step_up
However, when the target SIR is changed at every TTI (Transport Time Interval), control load on the radio network controller may be increased. Further, a bandwidth of an interface between the radio network controller and the radio base station may be largely consumed.
Accordingly, instead of changing the target SIR at every TTI, there has been known a method that sets a predetermined time section in which the target SIR is calculated by using the above-described algorithm at every time section, and notifies the calculated target SIR to the radio base station.
However, when the target SIR calculated based on this method is adopted, the quality is kept deteriorated for the above-described time section, due to the difference between the target quality and the set target SIR. Especially, when a link environment is changed, such as before or after a SHO (Soft HandOver) is performed, the communication quality required based on BLER (block Error Rate) and the target SIR that should be set may be totally different. There has been a problem that the communication quality for this time section can be largely deteriorated at this time, since the target SIR is kept unchanged.